It was Spock's way of acknowledging the start of the young crew's mission together/reliving the start of his mission with his Kirk and crew. It was hope and nostalgia mixed together. Spock wanted to ensure that his younger counterpart and young Kirk and the crew he knew would be on the job in this timeline as well... that's why he worked young Spock first, then went to witness Kirk's ascension to Captain.

"Thrusters on full..." was his way of acknowledgint that he'd gotten everyone together and that it was their turn now... their mission was underway.

It was actually Leonard Nimoy's line (he came up with it, and asked Abrams if he could say a line there when he was initially meant to be silent). Abrams mentions this on the DVD commentary, during the cave scene when Spock Prime meets Kirk. In the commentary it's revealed that he came up with the line while on set, and that it's "his blessing to the future of Star Trek."